(1) Field of the Invention
The present invention relates to a light collector for directing light to a concentrated area on a target surface and, in particular, to a linear light collector for an LED array used in an erase lamp in an electrophotographic printer.
(2) Description of the Related Art
In electrophotographic printing, a charge-retentive, photosensitive surface, known as a photoreceptor, is initially charged uniformly. In an exposure step, light either from an original image focused on the photoreceptor or created by the action of digital image data modulating a scanning laser selectably discharges specific areas of the surface to create a latent image. In a development step, toner particles are applied to the latent image. The toner particles adhere only to those areas on the photoreceptor on which an electrostatic charge remains. This developed image on the photoreceptor is then transferred to a print sheet on which the desired print or copy is fixed. After the developed image has been transferred, the photoreceptor is cleaned to remove any toner particles and paper fibers that may adhere to the belt.
In printing process outlined above there are various times, in addition to the exposure step, where it is desirable to discharge the photoreceptor. For example, prior to the transfer step, the photoreceptor is often discharged to reduce the holding charge on the toner and to improve the transfer efficiency. Similarly, during the cleaning step it is desirable to remove any and all residual charge on the photoreceptor prior to the next cycle.
One common method of reducing or removing the charge from the photoreceptor is to expose the photoreceptor to light. In xerographic equipment, what is commonly known as an "erase lamp" or "erase bar" is used to generate a small, well-defined band of light to discharge portions of the photoreceptor. An ideal erase lamp would be capable of selectably discharging the photoreceptor without generating stray light that may interfere with the operation of the printer. That is, as the performance of many of the elements of a xerographic printer, e.g., corotrons, optical sensors and imagers, are degraded in the presence of stray light, an ideal erase lamp would be efficient and direct substantially all of its light against the photoreceptor in a narrow band, with no unwanted stray light that requires attenuation.
A preferred light source for use in erase lamps is a light-emitting diode, or LED. When lit, an LED creates light that emanates from an area that is generally small when compared to the dimensions of the optic elements and thus creates a near point-source form. A collector is used to take the light from a series of small point-source LEDs and direct the beams therefrom so that the beams appear to be emanating from a single linear light source.
Several devices have been designed for directing light from an array of LEDs to a small linear area on a target surface. U.S. Pat. No. 4,767,172 discloses a collector for an LED array wherein each LED within the array is provided with a radially-symmetric collector, including a central convex lens portion and an outer parabolic reflecting surface. Light exiting substantially perpendicular to the substrate on which the LED is mounted is applied to the convex lens and collimated, while light exiting substantially parallel to the substrate on which the LED is mounted strikes a parabolic reflecting surface at greater than a critical angle thereof, and is also collimated, resulting in two concentric collimated beams which are directed onto a target surface.
U.S. Pat. No. 5,526,190 discloses an optical element adapted to be interposed between an array of light sources and a target surface. A central cylinder lens collimates light beams from the light source and directs the collimated light beams to a preselected area on the target surface. First and second outer reflective portions, each attached to and extending parallel to the cylinder lens, totally internally reflect light beams from the light source and direct the light beams reflected therein to the target surface.
The above-described devices collect light from an array of LEDs and direct the light to a small linear area on a target surface. However, one drawback associated with the devices described above arises from their shape. In both of the devices the central lens portion is lower in profile than the outer reflecting portion thereby creating a deep cavity or pocket over the central lens portion. These pockets tend to collect optical contaminants such as toner particles and paper fibers increasing the difficulty of the periodic cleaning required for all erase lamps.